The tests measure specific antibodies against SARS-CoV-2 in a blood sample. They can measure total antibodies or two subtypes, IgM and IgG, that occur in consecutive stages of the viral infection. The test is based on the reactivity to a viral protein immobilized in a surface where specific antibodies bind (in case they are present in the sample), which are detected by reagents that send out a signal (often a color).

They are distinct from diagnostic tests which detect and amplify RNA molecules of the virus in a sample collected from the respiratory tract, using the RT-PCR (polymerase chain reaction) technique.

Serology versus RT-PCR: Read the Smithsonian Magazine article.

For many infectious diseases the clinical trials used more often are the ones that test if there are antibodies in the patient’s blood capable of binding to the infectious agent (namely specific proteins), since this means that the patient has been exposed to this agent. However, there are distinct trials, as in the case of tetanus, where it is tested if the patient has antibodies that bind to the toxin produced by the infectious agent that causes the disease’s symptoms.

In the case of the SARS-CoV-2 virus that causes Covid-19’s disease, it’s thought that the antibodies that could give protection are those (named “neutralizers”) that block the interaction between components of the virus surface that form its “crown” (Spike protein) and the receptor in the surface of the cells (ACE2) that the virus uses as “lock” to enter human cells. The majority of tests developed to detect the exposure to SARS-CoV-2 virus are based on the detection of these antibodies.

Several tests with differing sensitivity and specificity levels, were developed to determine who is able to produce antibodies that recognize SARS-CoV-2, among the population.

Some of the tests are as simple and practical as a pregnancy test, but instead of detecting the presence of a hormone in urine, they detect the presence of specific antibodies against the virus in a drop of blood. These tests do not assess the amount of antibodies, nor evaluate the ability to neutralize the virus. Even though little insightful and of variable reliability, the qualitative results (negative or positive) are available in less than an hour.

Other tests, enzyme-linked immunosorbent assays (named ELISAs) have greater sensitivity and assess the antibodies specificity more effectively. Although slower, taking 3- 6 hours to obtain results, ELISA tests are quantitative and more insightful

The tests available vary in their performance, assessed in terms of specificity and sensitivity, in spite of the list of available tests or tests under development being constantly expanded, specially in USA and China. Spain and UK governments, have had great difficulties in finding a reliable test to implement at a national level.

Commercial tests of both types will have a high demand worldwide and will be subject to the laws of demand and market prices. ELISA tests, performed by public laboratories or academic institutions are accessible and easy to implement. These tests can be worthwhile as they are not subject to supply shortages. Besides that, they can be improved at several levels, namely regarding the virus protein to which the antibodies are directed to. They are quantitative and subject to a rigorous control quality process. They can also be associated to robotized methods, able to evaluate the function of the antibodies that have been produced. These type of tests are being produced by several institutions worldwide and will be crucial to expanding knowledge on SARS-CoV-2 virus in research projects.

However these commercial tests are expensive, when you intend to implement it at a large scale. The aim of our consortium is to obtain a test substantially cheaper but still top-quality, consistent with the methodology developed by Florian Krammer - with high specificity (reducing false positive) and  high sensitivity (reducing false negative) - that was aknowledged as valid by FDA (on 15 abril 2020). 

The consortium is keen in having an essay that recognizes SARS-CoV-2 Spike protein, initially through ELISA, but with the possibility of being transformed in a RDT.

Our test is based on the principles and materials granted by Florian Krammer that are being used worldwide.

It will be similar to the one developed by Stanford Medicine in the USA.

Our goal is to produce a test that allows SNS (Portuguese National Health System) to collect critical data using it epidemiologically to monitor Covid-19 mitigation strategies.   

Serological tests can be in a short term a cheap and quick way of monitoring the evolution of the disease’s transmission in the population, as it allows us to know who has been infected.

By confirming, as it is expected, that individuals with specific antibodies against SARS-CoV-2 are immune, the tests will be extremely important to determine which patient and individuals exposed to patients, become resistant (or not) to new infections. On the other hand, by repeating serological tests in the same individuals, we can know how long the presence of antibodies and immunity lasts.

The information on antibodies will be essential to test the efficiency of potential vaccines. The best way to stop the pandemic is through a vaccine. Serological tests will be essential for example to understand if a specific vaccine doses is enough to induce a lasting immunity, or if the vaccine has to be administered periodically.  

When someone is infected his body activates several defense mechanisms to control the infection. This process is called immune response. One of the response mechanisms is the production of antibodies, proteins that bind to the infectious agent – virus or bacteria – or to the toxins that it produces, which may lead to its elimination.

Antibodies circulate in the blood plasma, that reaches all parts of our body, and therefore can be detected in blood samples. Their concentration in the blood as well as their ability to neutralize the infectious agent in a test tube can then be determined by laboratory tests.

SARS-CoV-2 virus, which causes Covid-19’s disease, infects and replicates inside human cells. To fight the virus inside the cells the immune system has an additional response mechanism to antibodies. This response is generated by lymphocytes that recognize cells infected by the virus and kill them, inhibiting the replication of the virus.

During viral infection, there is antibodies and lymphocytes production. This response is necessary to fight infection, but it is also a response that induces physiological stress causing symptoms associated with the disease, like fever.

The immune system has memory. Commonly a person infected by a virus that has recovered of this infection, stays resistant to the virus and will suffer less with a second infection. In this case, we say that this person is immune to the virus, because she has antibodies and lymphocytes able to eliminate it. In some infectious agents, like measles or variola, immunity lasts all life. However, in the majority of common colds, caused by other types of coronavirus, the memory doesn’t lasts so long. Besides clinical trials of plasma of individuals who recovered from Covid-19 indicating that that is an immune response we still don’t know how strong or lasting the immunity is. This topic is being highly investigated worldwide through the study of individuals who had Covid-19 and recovered.

Vaccination is based on the immune memory. Normally vaccines involve an injection of proteins from the infectious agent or of the mitigated agent (there is, a version of the infectious agent transformed in the laboratory no to cause disease). Some modern vaccines, still being researched, use the genetic code of the infectious agent to command body cells to produce components of this agent leading to an immune response.

We need to know how effective is the immune memory to SARS-CoV-2 in order to know how effective and lasting can be the effect of vaccines that are under development.

Antibodies against SARS-CoV-2 components begin to be detected in the first week after the first symptoms start showing. However, the response can be slower in some patients than in other whose antibodies were detected in the plasma 10 days after the symptoms. One peculiarity of Covid-19 is that many infected individuals do not show symptoms. In order to clarify this issue the outcomes of longitudinal studies that follow the same individual throughout time are essential.

The presence of antibodies against SARS-CoV-2 in a serological test indicate that this person has been exposed to the virus. This information can be useful, once if the person didn’t have any symptoms, she now knows that she is probably asymptomatic. However, the knowledge we have in this moment doesn’t allow us to say this person is immune, since it is necessary to know if the antibodies are efficient in blocking the virus from infecting new cells. That is why it can be important to be able to measure additionally if blood samples have antibodies that can block the virus from replicating using cell cultures with living cells. These tests are more laborious and difficult to implement. Usually these tests are not used in medical practice, but are used by scientists that want to get to know the mechanisms involved in virus replication and the details of immune defense.

Reinfection cases have been reported (for instance in Japan and South Korea), but there is reason to suspect that it can be due to patients that never stopped being infected.

In fact, a factor that can mislead when studying reinfection is the delay of some patients (of several weeks) to sort out completely the primary infection. In the final stage of infection, the virus detection can be hazardous, leading to a new positive PCR result (after falling previously to detect it) and inducing the interpretation of reinfection.